1. Field of the Invention
The present invention relates in general to epitaxial reactors and, more particularly, to an improved induction heated pancake epitaxial reactor suited for chemical vapor deposition of an epitaxial layer of silicon upon a single crystal silicon wafer.
2. Description of the Prior Art
Heretofore, inductively heated pancake epitaxial reactors have been employed for chemical vapor deposition of n or p doped epitaxial layers of silicon upon silicon wafers. Such reactors have commonly employed an annular horizontal susceptor, as of graphite, coated with a protective layer of silicon carbide. The susceptor had an array of recessed pockets in its upper surface to receive the silicon wafers with their major faces, to be processed, lying in the horizontal plane. The susceptor and wafers were heated to a silicon deposition temperature, as of 900.degree. C. to 1200.degree. C. by means of a water cooled R.F. coil disposed below the susceptor and driven with R.F. current.
The susceptor, R.F. coil, and wafers were enclosed in a bell jar system including a quartz bell-shaped cover having a lower outwardly flanged lip which was selectively clamped to an elastomeric sealing ring carried on the upper face of a base plate for partitioning the interior of the bell jar system from the atmosphere of its surrounds. Reactant gases were fed at relatively high velocity into the bell jar through a vertical quartz tube centrally and coaxially disposed of the susceptor. The bell-shaped cover had a length greater than its diameter with a preponderance of its length protruding above the wafers as supported on the susceptor so that the vertically directed reactant gases at high velocity could impact upon the hemispherical end of the bell jar to mix thoroughly in a mixing zone above a deposition zone proximate the wafers. Spent reactant gasses were exhausted from the deposition and mixing zones of the bell jar system downwardly around the outer periphery of the susceptor.
Some of the problems encountered in such a prior art epitaxial reactor include: (1) slip was produced in crystalline wafers because of an excessive radial thermal gradient in the wafers caused by convective and conductive heat transfer to the silicon wafers and due to loss of thermal radiant energy from the susceptor and wafers to the surrounds of the bell jar; (2) the wafers were auto-doped by out diffusion of dopant and release of dopant by HCl etch from the wafers to the relatively high velocity turbulent reactant gases followed by incorporation of the dopant back into the deposited epitaxial layer; (3) silicon was deposited from the high velocity reactant gases onto the inside surface of the quartz bell jar cover, which deposits flaked off as particulates to contaminate the wafers and to require frequent cleaning of the bell jar cover; (4) the water cooled R.F. induction coil was disposed in gas communication with the reactant gases and SiO.sub.2 was deposited upon the coil structure requiring frequent cleanings; (5) the water cooled R.F. coil was disposed in substantial heat exchanging relation with the heated susceptor resulting in a substantial loss of heat to the coil and its coolant thereby increasing the operating R.F. power requirements of the reactor; (6) the sealing flange at the lip of the bell jar cover was expensive to fabricate and often failed under stress from the mechanical clamps used to force the gas-tight seal between the flange and the base plate; (7) the R.F. coil was of square cross-sectional area making it difficult to adjust the turn spacing to vary the spatial distribution of R.F. power density coupled into the susceptor to obtain uniform heating of the susceptor; and (8) the operator was not properly shielded from the flying debris resulting from a catastrophic failure (explosion) of the bell jar cover.
A conventional prior art inductively heated pancake susceptor epitaxial reactor is disclosed in a text entitled: "Silicon Processing for the VLSI Era", published in 1986 by Lattice Press of Sunset Beach, Calif., pages 145-147.